The present invention relates to a method for contactless measurement of a relative position of a magnetic field source which produces a magnetic field and a magnetic field sensor in relation to each other. Furthermore, the present invention also relates to a corresponding displacement sensor.
In particular, linear movements are intended to be detected and evaluated by means of the method according to the invention in a contactless manner by means of magnetic interaction between one or more permanent magnets and a magnetic sensor based on the Hall effect.
The measurement of linear movements is used, for example, for controlling machine tools, in pneumatics, in automation technology and robotics as well as in the automotive sector. Contactless detection of movements affords inter alia the advantage of freedom from wear. Among the contactless measuring methods, optical and magnetic methods are most widespread. Whereas the optical methods ensure a very high level of precision owing to the short wavelength of the light, magnetic methods are far less sensitive with respect to contamination and damage, in particular in that magnets and sensor components can be completely enclosed in a non-magnetic hermetic casing.
Displacement sensor systems in which the position of a displaceable permanent magnet is established by means of a two-dimensional or three-dimensional Hall sensor are marketed by various manufacturers.
In order to detect the relative linear movements at a location, two magnetic field components which are perpendicular to each other are measured and their quotient is evaluated in order to determine the position. This method has the advantage that, in regions in which a field component takes up an extreme value and therefore does not detect small displacements, the other field component reacts all the more strongly to displacements so that an approximately identically high level of measurement precision is provided over the entire measurement range.
Furthermore, this principle has the advantage that it is comparatively not very sensitive with respect to a change in the absolute magnetic field strength because proportional numbers between the field components are used to detect the position.
European patent specification EP0979988 B1 discloses measurement methods for contactless magnetic detection of relative linear movements between permanent magnets and electronic sensors. In order to detect the relative linear movements by means of the electronic sensors, two mutually perpendicular field components whose quotient is evaluated to establish the position are detected.
In a second method variant, the known measurement method can also be carried out so that two mutually perpendicular field components whose quotient is evaluated to establish the position are detected at two locations in order to detect the relative linear movements by means of the electronic sensors.
The published European patent application EP2159546 A2 discloses a measurement method for contactless detection of relative linear movements between a sensor arrangement for detecting two mutually perpendicular magnetic field components (R, A) and a permanent magnet. A two-dimensional or three-dimensional Hall sensor is used in place of individual sensors in order to detect different field components. The quasi-linear position measurement line is formed by the function U=y−e+g, where y is the functional relationship of the field components and e and g are predeterminable voltage values. In particular, a quasi-linear position measurement line U=f(y) is formed from the output signals of the Hall sensor according to the relationship y=a+b·R/f(c·Rn+d·An), where R is the radial field component, A is the axial field component, U is the measurement voltage and a, b, c, d and n are constant factors.
The published European patent application EP1243897 A1 relates to a magnetic displacement sensor which comprises a magnetic field source and a magnetic field sensor which can be displaced relative to each other along a predetermined path. The magnetic field sensor measures two components of the magnetic field produced by the magnetic field source. A position signal which represents the relative position of the magnetic field sensor and the magnetic field source is then derived from the measured components. The configurations of the displacement sensor shown in this specification are distinguished in that the determination of the position signal contains a division of the two measured components of the magnetic field.
However, those known methods have the disadvantage that the spacing between the permanent magnet and the magnetic sensor constitutes a significant error source in the measurement. It is often very difficult to keep that spacing constant, particularly owing to assembly tolerances, thermally induced material expansion and vibration influences.
FIG. 1 shows an arrangement in which a Hall sensor 100 is fitted so as to be fixed in position in order to contactlessly detect a linear movement and detects the magnetic field of a movable permanent magnet 102. In accordance with the north/south polarisation in the direction of movement of the permanent magnet 102, the magnetic field extending in the direction of movement is referred to below as the magnetic field component Bz and the component extending transversely thereto is referred to as the component By. The angle α, which can be calculated in accordance with the following equation (1) is generally used as the measurement signal.
                    α        =                  arc          ⁢                                          ⁢                      tan            ⁡                          (                              Bz                By                            )                                                          (        1        )            
As illustrated in FIG. 2, the angle α is dependent in a comparatively linear manner on the position of the permanent magnet 102 in relation to the Hall sensor 100 up to a given threshold value. Usually, the characteristic line currently being measured is further linearised, as illustrated in FIG. 2 by means of the line 104. That linearised line α_lin 104 then forms the characteristic output line of the sensor.
With reference to FIGS. 3 and 4, the dependence of the output signal on the spacing d between the permanent magnet 102 and the sensor 100 is explained in greater detail (see FIG. 1).
In FIG. 3, the groups of lines for the magnetic field components By and Bz are indicated with different spacings b as parameters. The line having the smallest magnetic field in terms of value is the one having the greatest spacing (in this instance, 7 mm), respectively. If the angle is calculated from those field components by means of its dependence on the position of the permanent magnet, the path illustrated in FIG. 4 is obtained. In order to illustrate how great the gradient error is, the characteristic lines are subtracted one from the other and the percentage error is set out. The dependence illustrated in FIG. 5 is produced as a result.
The line 106 represents the error between the lines for a spacing of 7 mm and the line for a spacing of 6.5 mm, whilst the line 108 represents the difference between the angle α for a spacing of 6.5 mm and the angle for a spacing of 5.5 mm. That is to say, a spacing variation of 1 mm results in a deviation of 1.6% and a spacing variation of 1.5 mm results in a deviation of 2.3%.
However, this is not acceptable for very sensitive systems such as, for example, an assisted steering system or a pedal travel measurement in a motor vehicle, so that there is a need for an improved signal processing operation which is less sensitive with respect to an undesirable variation of the spacing between the permanent magnet and the Hall sensor element.